Limiting the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and shelf life of many products. For instance, by limiting the oxygen exposure of oxygen-sensitive food products in a packaging system, the quality of the food product can be maintained and spoilage retarded. In addition, such packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and having to restock.
In the food packaging industry, several techniques for limiting oxygen exposure have been developed. Common techniques include those where oxygen is consumed within the packaging environment by some means other than the packaged article or the packaging material (e.g., through the use of oxygen scavenging sachets), those where reduced oxygen environments are created in the package (e.g., modified atmosphere packaging (MAP) and vacuum packaging), and those where oxygen is prevented from entering the packaging environment (e.g., barrier films).
Sachets containing oxygen scavenging compositions can contain ferrous compositions, which oxidize to their ferric state, unsaturated fatty acid salts on an absorbent, and/or a metal-polyamide complex. The disadvantages of sachets include the need for additional packaging steps (to add the sachet to the package), the potential for contamination of the packaged article should the sachet break, and the danger of ingestion by a consumer.
Oxygen scavenging materials also have been incorporated directly into the packaging structure. This technique (hereinafter referred to as "active oxygen barrier") can provide a uniform scavenging effect throughout the package and can provide a means of intercepting and scavenging oxygen as it passes through the walls of a package, thereby maintaining the lowest possible oxygen level throughout the package. Active oxygen barriers have been formed by incorporating inorganic powders and/or salts as part of the package. However, incorporation of such powders and/or salts can degrade the transparency and mechanical properties (e.g., tear strength) of the packaging material and can complicate processing, especially where thin films are desired. Also, these compounds as well as their oxidation products can be absorbed by food in the container, which can result in the food product failing to meet governmental standards for human consumption.
Oxygen scavenging compositions that include transition metal catalysts and ethylenically unsaturated hydrocarbon polymers which have an ethylenic double bond content of from 0.01 to 10 equivalents per 100 grams of polymer are known. However, because these polymers are amorphous, they can be difficult to blend and process with film-forming semicrystalline polymers conventionally used to make flexible packaging materials.
The use of a transition metal and a photoinitiator to facilitate initiation of effective scavenging activity of ethylenically unsaturated compounds is known. Because of the limited compatibility of the scavenger polymer with the film forming polymer, the amount of scavenger polymer in the blend must be limited and the resultant composition is difficult to process.
The use of a transition metal catalyst and a copolymer of ethylene and at least one vinyl unsaturated alicyclic monomer, preferably vinylcyclohexene, is known to provide excellent oxygen scavenging properties. However, prior to polymerization the vinylcyclohexene monomer is difficult to handle and has a strongly disagreeable odor.
Ideally, a polymeric material for use in an oxygen scavenging composition should exhibit good processing characteristics, be able to be formed directly into useful packaging materials or have high compatibility with those polymers commonly used to make packaging materials, and not produce byproducts which detract from the color, taste, or odor of the packaged product. Optimally, a packaging material formed from such a composition can retain its physical properties after significant oxygen scavenging.